The so-called menthol soap manufactured by compounding menthol to toilet soap for the purpose of producing a cold or refreshing feeling in its use has long been known. This menthol soap, however, has several disadvantages: for example, needle crystals of menthol grow and come out to the surface of the menthol soap during the storage or use thereof, and if the menthol soap is allowed to stand in a wet state after its use, cracks are formed therein. Conventional menthol soap, therefore, is hardly marketable, and it has been a continuing and significant subject for those engaged in the production of soap to overcome the above described disadvantages.
An attempt to add peppermint oil to a milled soap base has been made in recent years (see Japanese Patent Application Laid-Open No. 76100/1982). In this case, the amount of peppermint oil added is not more than 0.6% and is very small as a menthol content. Thus it fails to give a sufficiently satisfactory cold feeling on the skin. Furthermore, the process of producing such peppermint oil-compounded soap is very complicated and cannot be considered to be of high practical value.
When a milled soap is allowed to stand in a wet state after its use, there is a tendency for cracks to develop in the soap as it is dried. This will lead to a reduction in its product value. It is known, as described in Hans E. Tschakert, Seifen-Ole-Fette-Wachse, 83, Nr. 21, 610-612 and Nr. 22, 651-654 (1957), that such development of cracks in soap can be reduced by adding an oleyl alcohol/ethylene oxide adduct and a lauryl alcohol/ethylene oxide adduct to soap base.
With a further advance in the study to prevent the development of cracks in soap, it has been found that addition of surface active agents compatible with soap to soap base is effective for that purpose.
It may be considered, therefore, that marketable menthol soap can be produced by adding surface active agents as crack-preventing agents in combination with menthol to soap base. However, it has been revealed that the anticipated result cannot be realized; that is, although the development of cracks in soap can be reduced, the growth of menthol crystals in soap during the storage or use thereof cannot be prevented.